Geologic Time Geology deals with time, both in an absolute numerical sense and in a relative before/during/after sense. Relative Dating: order/sequence.

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Presentation on theme: "Geologic Time Geology deals with time, both in an absolute numerical sense and in a relative before/during/after sense. Relative Dating: order/sequence."— Presentation transcript:

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Geologic Time Geology deals with time, both in an absolute numerical sense and in a relative before/during/after sense. Relative Dating: order/sequence of events, without attaching absolute dates to them, mostly from field relationships. Usually can’t be re-set. Absolute Dating: numerical, absolute dates, mostly from radioactive decay methods. Can be re-set or overprinted v 0039 of 'Geologic Time' by Greg Pouch at :30:33 LastSavedBeforeThis :28:43 07GeologicTime.ppt

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Relative Ages/Relative Dating > Principles Principles used in Relative Dating Cross-cutting relationships: if event C cross-cuts events B and D, C is younger than B and D –The granite cross-cuts lower sedimentary sequence –The upper sedimentary sequence cross-cuts lower seds and granite. Inclusions If there is a chunk of formation W in formation D, D is younger than W. This can be tricky, since W could be an inclusion in D, or W could be an intrusion into D. –The granite has inclusions of the lower seds, and there are inclusions of granite in the upper seds. For sedimentary rocks and volcanic layers (supra-crustal rocks): Original horizontality and Lateral Continuity Sediments are originally deposited in nearly horizontal layers, that are typically very extensive laterally. If sedimentary layers are not horizontal, you need to explain them. If a sedimentary layer (or sequence) is laterally discontinuous, you need to explain it. –Lower seds were tilted and eroded. Upper seds look undisturbed. Superposition In a sedimentary sequence, the younger stuff is on top. Consider desks. –The upper sediments are younger than the lower sediments. The Upper White Limestone is younger than the Upper Gray Shale. Faunal succession Fossils succeed one another in a definite and determinable order, and a time period can be defined and recognized by its fossil content. Used with assemblages rather than single species. Missing information: Many events occur but are not recorded. These often cause unconformities.

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Relative Ages/Relative Dating Incorrect principles Facies succession: Rock types succeed one another in a definite and determinable order, and a time period can be defined and recognized by its rock type Constant rates: processes operate at the same rate for all of geologic time, with no interruptions

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Radioactive Ages > Quick Introduction to Atomic Physics Atom are composed of protons, neutrons, and electrons. Protons and electrons are charged. Protons and neutrons are found in the nucleus. Atoms can gain or lose some electrons fairly easily, so the number of electrons in an atom reflects its chemical state, and is not an intrinsic property. Protons and neutrons are hard to gain or lose. The number of protons causes an atom to have certain chemical properties. The count of protons in a nucleus is its atomic number, which defines what element it is. (Helium is 2). The number of protons and neutrons in an atom is its mass number. (Helium is usually 4 or 3). 4 He, He-4, Helium-4, or He 4 The number of neutrons in a nucleus is roughly correlated with the number of protons. Atoms of an element can have different numbers of neutrons. Each different mass number is an isotope of the element. Some isotopes are stable, others are unstable. Unstable isotopes decay (convert to other isotopes, while conserving charge and mass+energy) at a definite and measurable rate, following an exponential function like A(T)=A 0 e -λT or A(T)=A 0 2 -T/H This rate of decay is often expressed as the half-life H. (Time required, on average, for half the initial number of atoms to decay). An isotope whose atoms can decay is radioactive. The isotope that decays is the parent, the isotope that results is the daughter. Decay is accomplished by 1) conversion of a neutron to a proton and an electron (beta); 2) emission of helium nucleus (alpha); 3) conversion of a proton to a neutron and a positron (anti-electron) (electron capture or positron emission); or 4) breaking the atom more or less at random (fission, rare).

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Radioactive Ages >Use of Radioactivity to Determine Ages If there are crystals containing the parent isotope P that retain daughter product D along with a stable isotope S of D's element, you can accurately find “the age” of the sample by measuring the ratios of P to S and D to S in several grains (next slide for details, or for LOTS of details) A pair of parent and daughter isotopes, or a set of such pairs, is called an isotopic system. Radioactive dating requires very accurate measurements, favorable half-lives, and a crystal structure that can hold the daughter element. What “the age” refers to depends on the mineral, but usually means when the mineral was last above some critical temperature, called the closure temperature. In general, it is fairly easy to date igneous rocks, especially granitic rocks that contain a lot of potassium, uranium, and thorium. Sedimentary rocks are generally very hard to date, and it’s hard to tell what a date on a metamorphic rock means. If the daughter element is retained in the mineral but not found in it at formation, all daughter-isotope is radiogenic (like 40 Ar in KSpar), you only need to measure parent and daughter isotopes. If the daughter element is retained in the mineral and is found in it at formation, some daughter-isotope is radiogenic and some is original. If P only decays into D, P+D (on an atomic basis) is constant after formation, and you can find P and D at any time in the past. If you can find grains containing varying amounts of P and D, you can do some math to find initial amounts and age. If the daughter product is not retained in the sample (C-14 is a common example), an age is estimated using the amount of parent now present and estimates of the amount present initially. There is no check if the system may have been contaminated.

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Radioactive Age > Math Let –t be time AGO, so t=0 is now, t=7 days is a week ago; –H is half-life; –D is the amount of daughter isotope D, D* the radiogenic amount of daughter isotope –P is the amount of parent isotope P –S is the amount of some stable isotope S, probably that is of same element as D P, D, and S are in atoms or moles. t and H are in the same units, usually years. Radioactive Decay 1)P(t)=P Now * 2 +t/H There was more P in the past 2)D(t)=D Now – D* = D Initial + ( P Now *2 +t/H -P Now ) = D Initial + P Now *( 2 +t/H -1) Daughter isotope = Starting_Amount + Amount_from_Decay 3)S(t)=S Now for all t Because S is stable, S was constant through time. Normalize 2) by dividing by the amount of stable isotope S (a mass spectrometer will measure the ratio of two isotopes much more easily than it will give an absolute measurement of one isotope) 4)D(t)/S= (D Initial /S) + (P Now /S) * ( 2 +t/H -1) Suppose you have two specimens, A and B, for which the initial ratio of D to S was the same, and they lock in the daughter isotope (start the radioactive clock) at the same time. (D/S) A means the ratio of D to S in A 5)In A (D/S) A =(D Initial /S) A=B + (P/S) A * ( 2 +t/H -1) 6)In B (D/S) B =(D Initial /S) A=B + (P/S) B * ( 2 +t/H -1) We can measure D/S and P/S in A and B, and someone else has measured the half-life H very accurately. (The bolded terms were our measurements) Since D Initial /S is the same in A and B, we're left with two equation in two unknowns (D Initial /S) A=B and ( 2 +t/H -1), and from that and the half-life H we can get t and (D/S) Initial

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Radioactive Age > Graphs If you measure the amount of stable isotope and the present ratio of parent to stable in sample A, you can calculate the amount of parent and the ratio of P to S in A at any time in the past. If you measure the ratio of daughter to stable, you can then calculate the amount of daughter in sample A at any time in the past. This does you little good. However, if you do this for two samples, the trajectory of daughter-to-stable_A crosses the trajectory of daughter-to-stable_B at one point, which gives the age of the specimen and its initial ratio of daughter-to- stable isotope. RadioactiveDecay.xlsRadioactiveDecay.xls In reality, multiple specimens are analyzed and a least-squares fit is found.

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Radioactive Ages -> Good Bad and Ugly Good systems –Parent is common in common minerals, –Good half-life (10Ma to 1Ta), –Easy to measure daughters and parents Generally, ages can be found for minerals that contain the element of interest as a primary constituent or as a substitution. You usually measure many mineral grains in a rock: if they match, you have a single date. If they differ, you usually get a date of formation and a date of last major heating. –K-40 -> Ar-40 works very well in feldspar, other minerals with potassium –Rb-87 -> Sr-87 works well in feldspars (Rb behaves like Na, Sr behaves like Ca) –Th-238->Pb-208 –U-238 -> Pb-206 –U-235 -> Pb-208 Bad systems An isotopic system can be bad because the isotopes are rare, has a bad half-life (too short or too long), it is difficult to measure daughters or parents, its concentration depends on external factors, or daughter products are not retained. Cosmogenic isotopes (produced from cosmic rays) work poorly, but are often the only ones available for recent materials. (Asserts that cosmic rays happen at constant rate over long term but not short term.) –C-14 is the most famous bad system. It is hard to determine parent originally present, the daughter product is not retained, the system is easily reset by later events, such as bacterial decay and incorporation of old (dead) carbon or isotopic fractionation can alter the initial value. Stable Isotopes 99StableIsotopes.ppt99StableIsotopes.ppt

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Other absolute age techniques In addition to radioactive techniques, there are a few other ways to obtain absolute dates on a geologic events. One that occasionally works is historical records. Counting annual tree rings or annual layers of sediment (grad student work) can provide ages. Many organic compounds exist in two mirror-image forms (enantiomers). Organisms produce/consume only one of them. With time (and heat), the biologically favored can alter into its mirror image. It is sometimes possible to estimate an age based on the relative abundance of the two enantiomers, if you like the assumption it's been at a constant temperature and isolated. New materials are entirely L or R, old materials are 50% R and 50% L. If the source of magmas or sediments has a known abundance of radioactive isotopes and very homogeneous composition, the amount of daughter isotopes in the magmas or sediments should vary with time, and this can be used to estimate a time of extraction from the reservoir. Very sensitive to departures from the assumption of homogeneity and fractionation.

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Calibrating Relative Ages with Absolute Ages By examining fossil assemblages, geologists were able to construct a standardized, relative time scale that is used on a worldwide basis, mainly for the time during which there have been abundant fossils. Occasionally, an igneous event has a cross-cutting relationship with a sedimentary rock layer at one of the boundaries that allows an absolute age from the igneous event to be associated with the relative age from the sedimentary layer. The odds of such a fortuitous combination are low, and there are relatively few absolute dates associated with time boundaries. Most of the subdivision is done by interpolation between these known points. Because it relies on fossils, it turns out to be very finely sub-divided for recent geologic history and coarsely divided for older events. Named periods are based on big changes in fossil assemblage. It turns out that many boundaries are located at times of slow- or non- deposition, resulting in a fairly normal extinction rates looking more dramatic than they are.

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Correlation Establishing the equivalence (age and original continuity) of rocks in two areas. This is important for unraveling geologic history, mineral deposits, water supply, oil prospecting, predicting what rock units will be found where (horizontally and vertically)…. Most of the reasons people do geology involve the reliability of correlations. Correlations can be of either time or original continuity, sometimes both. How do we correlate? –Tracing the unit is the only really reliable method –Error-prone. Lithologic correlation If the rock types (facies) are identical or similar, assert the rocks must correlate. Can also use sequence, which can help, but facies often succeed each other in a definite order. –Fossil correlation Using the fossils present in a rock layer (if there are any), you can correlate the time of deposition. Gives time units and does not help with original continuity. Time resolution can be as fine as Ka under good circumstances, or as coarse as 100 Ma in the Phanerozoic or 1Ga in Precambrian. –Absolute ages Where an absolute age from radioactive techniques is available, they can be used to determine the sequence of events, if they are precise enough. –Chemical correlation: compounds or elements found in unusual concentrations or combinations can be used as a sort of fingerprint. –Lithologic sequence of unrelated facies.

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Unconformities A conformable sequence has sediments and volcanics deposited continuously and one after another with no pauses. Any deviation is an unconformity. Paraconformity sedimentary rocks overlying sedimentary rocks with a period of non-deposition (a hiatus) separating them. Very hard to recognize. Disconformity sedimentary rocks overlying older, eroded sedimentary rocks, indicating a period of erosion. Note that you can have deposition followed by erosion. Angular unconformity a change in the bedding angle that indicates a period of deformation and erosion Nonconformity eroded igneous or metamorphic rocks overlain by sedimentary rocks.

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Unconformity Pictures

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Geologic Dating A geologic event can be before/during/after relative to some other event. This is relatively easy, at least to bracket. Absolute dates can be assigned to geologic events using the relative amounts of radiogenic daughter product and parent isotope. This requires luck and lots of instrumentation. A conformable sequence of rocks is sediments+volcanics laid down one after another with no breaks in deposition. Anything else in an unconformity. (Nonconformity, angular unconformity, disconformity, paraconformity) Rock units are mappable bodies of originally continuous material. They are correlated from location to location by –Walking the outcrop –Similarity of rock type and sequence –Chemical and physical measurements –Paleontology